EP4043125A1

EP4043125A1 - Porous and cellular metals and metal structures of open porosity impregnated with cork, production processes thereof and uses of same

Info

Publication number: EP4043125A1
Application number: EP20811435.5A
Authority: EP
Inventors: Isabel Maria ALEXANDRINO DUARTE; Susana Cristina DOS SANTOS PINTO; Paula Alexandrina DE AGUIAR PEREIRA MARQUES
Original assignee: Universidade de Aveiro
Current assignee: Universidade de Aveiro
Priority date: 2019-10-11
Filing date: 2020-10-12
Publication date: 2022-08-17
Also published as: WO2021070155A1; PT2021070155B

Abstract

The present invention refers to porous and cellular metals and metallic structures of open porosity embedded with strengthened or non-strengthened cork, manufacturing processes and uses thereof.

The process for obtaining these new materials and structures comprises filling the open pores of porous and cellular metals and metallic structures with a cork-based material in the form of particles, granules, grains, powder, and the like and combinations thereof, obtainable from cork of several types and origins.

The developed materials are light-weighted, multifunctional, have improved properties of acoustic insulation and thermal behaviour, and a significant enhancement of mechanical performance.

The present invention pertains to the field of macromolecular organic compounds and the preparation thereof, in particular preparation of macromolecular compounds for porous or cellular materials, such as foams of composite materials, by the embedding process, with applications in the construction industry, transportation, furniture and design, footwear, construction of machines, tools and devices.

Description

Technical field of the invention

The present invention refers to porous and cellular metals and metallic structures of open porosity embedded with strengthened or non-strengthened cork, manufacturing processes thereof and uses thereof.
The process for obtaining these new materials and structures comprises filling the open pores of porous and cellular metals and metallic structures with a cork-based material in the form of particles, granules, grains, powder, and the like and combinations thereof, obtainable from cork of several types and origins, such as natural, expanded, recycled cork, cork waste from industrial processing with different grain sizes and densities, cork by-products, which may contain at least one natural, synthetic, or recycled polymer that promotes bonding of cork with each other and with the porous metallic network, possibly containing micro- or nanometric-sized strengthening elements, and the like and combinations thereof, in order to improve the properties of the traditional porous and cellular metals and metallic structures of open porosity.
The new materials thereby obtained are light-weighted, multifunctional, have improved properties of acoustic insulation, thermal behaviour, and even a significant enhancement of mechanical performance.
For that reason they can be advantageously used in the construction industry, transportation, furniture and design, footwear, construction of machines, tools and devices, such as construction of houses, cars, trains, boats, aircraft, tools, machines, devices, furniture, design pieces, with good properties of acoustic absorption, improvement of thermal behaviour and mechanical performance, low weight with high stiffness and good absorption capacity of impact and shock energy.
In this way, the present invention pertains to the technical field of macromolecular organic compounds and preparation thereof, in particular preparation of macromolecular compounds for porous or cellular materials, such as foams of composite materials, by the embedding process.

Background of the Invention

Solid and porous cellular materials, such as foams and sponges, have become the most promising light-weighted and multifunctional materials, being used or tested in a wide range of commercial, biomedical, industrial and military applications. This is mainly due to the rare combination of the properties derived from their cellular structures formed by open or closed pores and the properties of the base (matrix) material from which they are made. Their use allows an immediate and significant decrease in weight, combined with other benefits, namely an excellent damping of noise and vibrations, acoustic attenuation, impact and shock energy absorption, good filtering capacity, catalytic properties and acoustic and thermal insulation.
Cork and wood, being examples of materials of natural origin, were the first building materials used by man in the construction of houses, kitchen and fishing and hunting appliances. Recently, a wide variety of cork products have been developed and marketed (Gil, 2019) due to their excellent properties derived from their cellular structure, such as low density, high friction coefficient, low thermal conductivity, high resistance to moisture and penetration of liquids, resilience and excellent vibration-absorbing capacity and compressibility.
Inspired by these natural cellular materials, man began to develop similar materials made from polymers, metals and ceramics, named synthetic cellular materials, also called artificial or bio-inspired materials (Duarte et al., Sci. Techn. Mater. 2018). Among these, porous metals are the ones with better properties for engineering applications. They are easily recyclable and extremely resistant, showing a plastic deformation that absorbs large amounts of energy and withstanding higher temperatures than cellular polymers. Furthermore, they are non-flammable, unlike known porous and cellular polymers.
In recent years new products based on these cellular and porous metals have emerged with the development of new processes or improvement of existing ones, aiming at extending their multifunctional character and improving their properties, allowing to control the distribution, size and geometry of cellular pores during their manufacture, an essential requisite to predict their operational behaviour, while minimizing production costs and residues/waste.
With rapid advances in additive manufacturing technologies, such as fast prototyping and 3D printing, new cellular materials have emerged with regular or periodic cellular structures of open porosity that can be easily characterized by a unit cell (Wadley, 2006).
Other cellular materials have likewise emerged, combining materials having a periodic or stochastic porous network with other materials. Metallic matrix syntactic foams, metallic hollow sphere structures and metallic foam-polymer hybrid sphere structures of closed porosity are examples of these materials with an easily reproducible unit cell.
Syntactic foams are manufactured by simple infiltration of a molten metal through a predefined arrangement of hollow spheres or porous particles made of ceramics, glass and metals, completely filling the empty spaces.
" US2017307137 (A1 )", " US2017307138 (A1 )" and " US2018099475 (A1 )" disclose cellular structures wherein each cell has a 12-cornered cross section, which are generally called honeycomb structures, whose geometry may vary widely but with the common feature of being formed by a matrix of hollow cells arranged between thin vertical walls. They are produced by joining cells via processes such as adhesive bonding (gluing), resistance welding, brazing, diffusion bonding or thermal fusion. All these production methods are based on how the connection between adjacent plates is made in order to form a "node" (connection point of different plates). In these documents, cork is generally referred to as a thermal insulating material or intended for applications where temperature control is an important issue. In addition, the structures disclosed in these documents share the walls with each other (hollow cells formed between thin vertical walls), the cellular structures to be filled described in these patents being formed by profiles/hollow cells joined by the thin walls using a bonding technique, the cellular structures being relatively large and with a small number of cells, wherein each cell can be separately manufactured by different processes and then joined, the materials forming the cellular structures varying extensively, such as, for example, steel alloys, titanium alloys, aluminium alloys, magnesium alloys, nylons, plastics, polymers, compounds, fibre-strengthened composites, silicone, semiconductors, papers, rubber, foams, gels, woods, stoppers, hybrid materials (i.e., multiple dissimilar materials), shape-memory materials and/or any other suitable materials, thereby providing them with a totally different technical effect from porous and cellular metallic structures of open porosity embedded with cork.
US20130098203A1 is an exemplary disclosure of this type of foam. These metallic matrix syntactic foams have much higher density values (> 1000 g/cm³) than conventional metallic foams of closed porosity (<900 g/cm³), thereby narrowing their applications.
Hollow metallic sphere structures are typically obtained by joining different hollow metallic spheres using a metal, polymer or polymeric foam (Andersen, 2000), and DE3724156A1 also discloses this process. The use of these structures is limited by the high production cost of these hollow spheres, making this process very uncompetitive.
Hybrid metallic foam sphere structures of closed porosity are manufactured by heating an empty mould or hollow structure containing small aluminium spheres of closed porosity, obtained by the powder metallurgy method, coated with a polymeric material, described in WO2005000502A1 . The simplification of the process, automated and continuous production are some of the advantages of these structures (Stöbener et al., 2009). However, this manufacturing process neither does guarantee totally perfect spheres, nor is their cellular structure controllable throughout their manufacture.
In order to overcome these limitations, metallic foams of open porosity embedded and completely filled with polymer have also been recently developed, as disclosed in WO2012072543A1 and WO2018087076A1 and Duarte et al., Polymer Testing, 2018. This type of process, using these periodical foams, allows predicting their mechanical properties and, at the same time, the mechanical resistance is guaranteed by the polymer. Although the results are promising in terms of mechanical properties, the use of polymers is the main disadvantage of these foams due to their high flammability that increases the risk of fire, with the release of toxic gases and smoke. In addition, the fact that some of these polymers are not recyclable and the recycling of others is not cost-effective makes their use very limited.
In this way, the present invention aims at providing an improved alternative to the above-mentioned state of the art materials by proposing porous and cellular metals and metallic structures of open porosity embedded with cork instead of polymer.

Summary of the invention

The present invention refers to porous and cellular metals and metallic structures of open porosity embedded with cork, either strengthened or not, manufacturing process thereof and uses thereof.
The porous and cellular metals and metallic structures of open porosity of the present invention form a matrix containing periodically or randomly arranged open pores with at least one cork material in the form of particles, granules, grains, powder, and the like and combinations thereof, and wherein said cork material is incorporated in the pores of the metallic material, as described in claim 1.
These new materials are multifunctional and have improved properties of acoustic insulation, thermal behaviour, and even a significant enhancement of mechanical performance. In addition, they are recyclable and non-flammable.
The process for obtaining these new materials and structures comprises filling the open pores of porous and cellular metals and metallic structures with a cork-based material, as described in claim 9.
This process has the advantage of allowing to control the cork powder dimensions, relative to the open pore sizes of the aluminium foam, with the possibility of recycling and reusing the cork products and cork industry residues/waste, thereby increasing their life cycle.
The flexibility, starting materials and low cost of the process of the present invention are also advantages relative to the existing processes since it is possible to use several by-products and no temperature is applied or, when it is applied it is always lower than the required temperature for other processes such as, for example, investment casting, melting, additive manufacturing, 3D printing, etc.
In this way, these new porous and cellular metals and metallic structures of open porosity embedded with cork are multifunctional, with better acoustic, thermal and mechanical properties than their individual components, providing the necessary characteristics to fulfil the requirements of a given specific application. The present invention also allows to recover the residues/waste from the cork industry and to "create" new applications for the porous and cellular metals and metallic structures of open porosity, i.e., also for structural applications.

Description of Figures

Figure 1 : Schematic representation of the manufacturing process of porous and cellular metals and metallic structures of open porosity embedded with strengthened or non-strengthened cork with nano- and micrometric-sized strengthening elements.
Figure 2 : Appearance and morphologies of cork granules (a) and aluminium cellular metal of open porosity (b), which are the main raw materials used for manufacturing cork agglomerates, and (c) cork-embedded aluminium foams.
Figure 3 : Compressive stress-strain curves, wherein:
- Fig. 3a shows the respective energy absorption curves,
- Fig. 3b shows the compressive stress-strain curves of cork-embedded aluminium foams and their respective individual components, where it is found that, in the aluminium foam of open porosity and cork agglomerate, the presence of cork inside the aluminium foams causes increases of 159% and 81% in mechanical resistance and energy absorption capacity, respectively,
- Fig. 3c shows the compressive stress-strain curves of aluminium foams embedded with graphene oxide-strengthened or non-strengthened cork and their respective individual components, namely aluminium foam of open porosity, cork agglomerate and cork nanocomposite agglomerate, where it is found that the presence of cork inside the aluminium foams causes increases of 395% in mechanical resistance for a strain of 0.6%, and
- Fig. 3d shows the compressive stress-strain curves of aluminium foams embedded with graphene oxide-strengthened or non-strengthened cork and their respective individual components, namely aluminium foam of open porosity, cork agglomerate and cork nanocomposite agglomerate, where it is found that the presence of cork inside the aluminium foams causes increases of 344% and 238% in mechanical resistance for a strain of 0.5% in a quasi-static or dynamic regime.
Figure 4 : Comparison of sound absorption and noise reduction coefficients, wherein:
- Fig. 4a shows a comparison of sound absorption and noise reduction coefficients for a cork-embedded aluminium foam and the respective individual components, namely aluminium foam of open porosity and cork agglomerate, where a significant increase in the sound absorption and noise reduction coefficients is found over a wide range of medium-high frequencies (1000 to 3000 Hz), the typical human frequency being comprised between 1000-2000 Hz,
- Fig. 4b shows a comparison of sound absorption and noise reduction coefficients for an aluminium foam embedded with graphene oxide-strengthened cork, referred to as cork nanocomposite, and the respective individual components, namely aluminium foam and cork nanocomposite agglomerate, where it is found that the aluminium foam embedded with cork nanocomposite exhibits a high sound absorption coefficient over a wide range of medium-high frequencies (1000-4000 Hz), with the value of 1 between 1700 Hz and 2000 Hz and a value higher than 0.85 between 1261 Hz and 4000 Hz,
- Fig. 4c shows a comparison of sound absorption and noise reduction coefficients for an aluminium and cork foam and aluminium foam embedded with graphene oxide-strengthened cork, referred to as cork nanocomposite, and the respective individual components, namely aluminium foam, cork agglomerate and cork nanocomposite agglomerate, where it is found that the aluminium foam embedded with cork nanocomposite exhibits a high sound absorption coefficient over a wide range of medium-high frequencies (1000-4000 Hz), with the value of 1 between 1700 Hz and 2000 Hz and a value higher than 0.85 between 1261 Hz and 4000 Hz.
Figure 5 : Comparison of thermal conductivity values for aluminium foams embedded with graphene oxide-strengthened or non-strengthened cork, with their individual components, namely aluminium foam of open porosity and cork agglomerate, where it is found that the former ([0.091 W/(m.k)] and 0.101 W/(m.k)]) present higher values than the samples of graphene oxide-strengthened ([0.057 W/(m.k)]) or non-strengthened ([0.055 W/(m.k)]) agglomerated cork.
Figure 6 : Sequence of images 2s from the flame test of a sample, wherein:
- Fig. 6a refers to a sample (25 × 25 × 25 mm) of agglomerate with graphene oxide (a) and a sample (25 × 25 × 25 mm) of aluminium foam embedded with graphene oxide-strengthened cork, referred to as cork nanocomposite, where it is found that the flame is extinguished faster in the aluminium foams of open porosity embedded with graphene oxide-strengthened cork, and
- Fig. 6b refers to a sample (25 × 25 × 25 mm) of cork agglomerate, a sample (25 × 25 × 25 mm) of aluminium and cork foam, a sample (25 × 25 × 25 mm) of graphene oxide-strengthened cork agglomerate, referred to as cork nanocomposite, and a sample (25 × 25 × 25 mm) of cork nanocomposite-embedded aluminium foam, where it is found that the flame is extinguished faster in the aluminium foams of open porosity embedded with graphene oxide-strengthened cork.

Description of the invention

The present invention refers to the development of new porous and cellular metals and metallic structures of open porosity embedded with strengthened or non-strengthened cork, comprising filling the periodically or stochastically arranged open pores with a cork-based filling material.

1. Porous or cellular metal or porous or cellular structure

Porous metals, cellular metals or porous or cellular structures are characterized by being mainly comprised of open, interconnected, edge-sharing pores, forming periodically or stochastically arranged three-dimensional arrays that encompass metallic foams and metallic sponges, and also metallic cellular structures that include periodic cellular materials with different topologies, namely but not limited to honeycomb, trellis or prismatic reticulates.
Materials of this type suitable for use in the scope of the present invention are, for example, metal open foams with pore sizes of 5 ppi to 500 ppi (conversion: internationally this represents m²) manufactured by the Investment Casting method, and metallic cellular structures manufactured by additive manufacturing technologies, such as fast prototyping and 3D printing of metals, preferably with a variable size of 1 ppi to 500 ppi, more preferably of 10 ppi to 250 ppi, even more preferably of 50 ppi to 150 ppi, or even of 75 ppi to 100 ppi.

2. Metallic matrix

The metallic matrix consisting of at least one metal or metallic alloy is, for example aluminium, magnesium, manganese, copper, silicon, zinc, tin, nickel and alloys thereof used for the different structural and functional applications of this type of metals such as, for example, acoustic and thermal applications and water filtering.

3. Cork-based filling material

Cork is a 100% natural material, of vegetable origin from the bark of cork oaks (Quercus suber), rich in suberin, a wax synthesized by the cells of cork oak's bark. It is highly hydrophobic, comprising essentially two reactive groups, a polyaromatic group and a polyaliphatic one, each consisting of monomers of hydroxycinnamic acids and derivatives such as feruloyltyramine, and monomers of α-hydroxyacids such as, for example, 18-hydroxyoctadec-9-enoic acid, and α,ω-diacids such as, for example, octadec-9-ene-1,18-dioic acid, respectively, providing cork with unique properties:

It is extremely light-weighted (e.g. natural cork: 160 - 260 kg/m³, granulated cork: 60 - 160 kg/m³, agglomerated cork: 140 - 600 kg/m³), smooth and pleasant to the touch with large elasticity and resilience, easily recovering its original shape after being subjected to pressure;
It is impermeable to both liquids and gases (it does not rot) preventing, for example the absorption of water;
It is very resistant to fire, does not flame or expel toxic gases during combustion;
It withstands large temperature and humidity variations;
It has low thermal conductivity, thereby being an excellent thermal insulator due to the air entrapped inside the cells;
It has high capacity of noise and vibration absorption, thereby being an excellent acoustic insulator;
It also has a good capacity of impact energy absorption.

Thus, cork is currently used for several purposes such as, for example, wine closures (stoppers for wine bottles), in footwear, furniture, decoration and design, in building construction for acoustic and thermal insulation on floors, walls, doors, windows, roofs, or for means of transportation (cars, aircraft, trains, boats, etc.). Cork is recyclable, reusable and non-flammable, like the most environmentally friendly materials.
In the scope of the present invention, the cork material may be used in expanded form, in the form of particles, grains, granules or powder with different densities and/or grain sizes.
This cork may come from recycled cork, cork waste from industrial processes, such as grinding and sanding dusts, or technical products such as floating dusts, among others, or from mixing more than one of the varieties of cork waste or cork by-products, such as cork stoppers, from expanded pure and black agglomerates, which may contain at least one polymer of natural, synthetic or recycled origin, such as polyurethane, silicone, epoxide, polyethylene, polypropylene, ethylene, alkyl or aryl anhydride, polystyrene and polycarbonate.
Quantitatively, the cork material can be used in amounts varying between about 3%-30% by mass of polymer of natural, synthetic or recycled origin according to the intended application, with about 0%-15% by mass of additives and about 0%-10% and 0%-50% by mass of nanometric and micrometric-sized strengthening elements, respectively.
Furthermore, the cork material can also be subjected to pretreatment, such as granulation processes, particle size-reduction processes such as cutting, grinding, milling, and size separation processes, such as sieving that promotes the separation and sorting by particle, granule or grain sizes.
Cork must have a size distribution smaller than the open pore size distribution of the porous and cellular metal and metallic structure to be used, in order to favour its incorporation in these pores.
In a preferred embodiment, the cork material has a variable size between the nanometric scale and micrometric scale.
The cork-based filling material can be strengthened by mixing the raw materials, which can be optionally functionalized by, for example, physical or chemical modification, such as removing impurities to promote a better adhesion between the various constituents, namely washing with one or more solvents and drying the cork residues, or using polymers previously functionalized with thermoset functional groups such as, for example, Epoxide and Polyurethane.
The non-strengthened cork-based filling material is obtained by simply mixing the cork, in the form of particles, granules, grains or powder, with at least one polymer, either functionalized or not, with the addition of processing additives, using mixing techniques such as, for example, mechanical stirring.
The strengthened cork-based filling material is obtained by using the common techniques of incorporating nano- and micro-sized strengthening elements with the addition of at least one polymer, either functionalized or not, of natural, synthetic or recycled origin, and with the addition or not of processing additives. For example, incorporating micrometric-sized strengthening elements in the cork using a mechanical stirrer, or incorporating nanometric-sized strengthening elements in the cork using the layer-by-layer technique; to this end aqueous solutions can be used.
In a preferred embodiment, the filling material is a strengthened one.
Strengthening elements for the cork material suitable for the present invention are materials dispersed in the continuous matrix, which encompass micro- and nano-scale fibres, particles, tubular structures or sheets of polymers, ceramics, glass and metals, carbon-based materials, and the like and combinations thereof. In case the strengthening element is micrometrically sized, it is called a composite material, or simply "cork composite". In case the dispersed phase is nanometrically sized, it is called a nanocomposite material or simply "cork nanocomposite".
Polymeric materials, or simply "polymers" are materials of natural, synthetic or recycled origin, and the like and combinations thereof, which can be divided in thermoplastics, thermoset and elastomers according to their mechanical characteristics. Thermoplastics include the known plastics, which can be melted several times and in some cases dissolved in various solvents, thereby being recyclable materials. Thermoset are stiff and fragile polymers, being very stable to temperature variations, although heating these polymers causes them to decompose before melting and therefore are not easy to recycle. Elastomers have a high elasticity and are neither rigid nor meltable, reducing the recycling ability.
In a preferred embodiment, the filling material is strengthened with one or more of the following epoxide- or polyurethane-type of polymers.
Suitable additives for this purpose are substances used in small amounts employed for modifying and/or improving various properties, namely promoting processability, providing thermal stability, colouring, improving anti-static properties, surface hardness and fire resistance, among others.
In a preferred embodiment, the strengthened or non-strengthened filling material comprises one or more additives.

4. Metallic matrix strengthening elements

Strengthening elements can be of natural, synthetic or recycled origin, namely but not limited to ceramics, polymers and metals, carbon derivatives, for example, graphene oxide, in the form of micrometric- or nanometric-sized tubular structures, sheets, particles and fibres, and the like or combinations thereof.
These new light-weighted and multifunctional porous and cellular metals and metallic structures of open porosity embedded with strengthened or non-strengthened cork may be an integral part of the usual transformation processes of several companies related to the industry of porous and cellular metals and metallic structures of open porosity and the cork industry.

5. Process for obtaining porous and cellular metals and metallic structures of open porosity embedded with cork

The present invention also refers to the manufacturing process of these new porous and cellular metals and metallic structures of open porosity embedded with strengthened or non-strengthened cork, which is schematically depicted in Figure 1.
After the raw materials are selected as a function of the desired final material and as previously described, the cork-based filling material, either strengthened or not, is prepared also depending on the desired final material. The open pores of a cellular or porous metal or structure are then filled with the cork-based filling material. Finally, the resulting material is densified and cured.
The filling material is prepared by mixing the various selected components, namely cork, the polymer, in case the filling material is to be strengthened, and possibly adding processing additives and strengthening elements.
The mixing is carried out in a single phase, i.e. by promoting the simultaneous contact of all the selected components, at ambient conditions with strong mechanical stirring.
In case a strengthened cork filling material is to be obtained, the strengthening elements may be incorporated using dry and/or wet techniques, such as mechanical stirrers, colloidal processing and step-by-step technique, and the like and combinations thereof, selected according to the chemical and physical characteristics, shape, geometry and size distribution of these strengthening elements.
The step of filling the open pores of porous and cellular metals and metallic structures is performed by pouring the filling material, prepared as previously described, into the periodically or randomly distributed open pores of a porous or cellular metal or metallic structure, preferably under vibration and/or pressure in order to ensure that the pores are filled.
For this purpose, thin-walled moulds or hollow structures with simple and complex geometries can be used, optionally employing coatings with non-stick properties such as tracing paper, Teflon sheets, Kraft paper, silicone-based paper and films, to favour the extraction of the resulting new materials.
Finally, the material resulting from the described steps is subjected to a densification and curing step performed under controlled conditions, either in air or in vacuum, preferably in air with no temperature applied or also possibly varying the temperature, typically up to 250°C.
To that end, said material, formed by a metal framework with the pores at least partially filled with a cork-based material, is pressed by exerting pressure up to the required density for each application, but ensuring that the initial porous and cellular metal or metallic structure of open porosity is not damaged.
This is followed by the curing step, namely the hardening of at least one polymer material by cross-linking, whose onset can be induced by chemical additives (e.g. water), heat or ultraviolet light.
Finally, the resulting material, a porous and cellular metal or metallic structure embedded with cork is extracted from the mould.
In summary, the production process of porous and cellular metals and metallic structures of open porosity embedded with cork of the present invention comprises the following steps:

a) Preparation of the cork-based filling material;
b) Filling the open pores of a cellular or porous metal or structure with the filling material of (a);
c) Densification and curing of the material from (b).

Definitions:

In the context of the present invention, the percentages mentioned in the present description and claims refer to mass percentages.
In the context of the present invention, the term "comprising" is to be understood as "including, among others". As such, said term should not be construed as "consisting only of".
Note that any value X presented throughout the present description is to be interpreted as an approximate value of the real value X, as such an approximation to the real value would be reasonably expected by the one skilled in the art due to experimental and/or measurement conditions that introduce deviations from the real value.
In the context of the present invention, the ranges of values appearing in the present description are intended to provide a simplified and technically acceptable way of indicating each individual value within said range. By way of example, the expression "1 to 2" or "between 1 and 2" means any value within this range, including the limit values. All the mentioned values are to be interpreted as approximate values. For example, reference to "0.1" means "about 0.1".
In the present invention, the term "cellular metal" refers to porous, solid metals characterized by being mainly comprised of open pores, encompassing metallic foams and metallic sponges obtained by standard processes, and also metallic cellular structures including periodic cellular materials with different topologies, namely but not limited to honeycomb, trellis or prismatic reticulates, primarily obtained by additive manufacturing technologies, such as fast prototyping and 3D printing.
In the present invention, the term "cellular metal of open porosity" refers to solid porous metals mainly comprised of open, interconnected, edge-sharing pores, forming periodically or stochastically arranged three-dimensional arrays.
The number of these pores that make up one inch is called "pores per inch", PPI and is a way to characterize the pore size of a cellular metal of open porosity.
In the present invention, the term "metal" refers to metals and metallic alloys thereof, namely but not limited to copper and alloys thereof, manganese and alloys thereof, aluminium and alloys thereof, magnesium and alloys thereof, zinc and alloys thereof.
In the present invention, the term "porous and cellular metallic structure" encompasses preferably periodic cellular structures with different topologies, namely but not limited to honeycomb, trellis or prismatic reticulates, primarily obtained by additive manufacturing technologies such as fast prototyping and 3D printing.
In the present invention, the term "porous and cellular metal embedded with strengthened or non-strengthened cork" refers to materials typically comprised of porous and cellular metal formed by a network of periodically or randomly arranged open pores, whose inner spaces/voids are filled with a cork-based filling material in the form of cork particles, cork grains, cork granules, recycled, natural cork powder, cork waste from production processing or from a by-product or product of cork, and the like and combinations thereof, and at least one natural, synthetic, recycled polymer, which may contain nano- or micro-sized strengthening elements, and the like and combinations thereof, in order to modify and improve thermal and acoustic properties and mechanical performance, whose combination provides the product characteristics.
In the present invention, the term "strengthening element" refers to materials dispersed in a continuous matrix, in this case the cork-based filling material, encompassing micro- and nanoscale fibres, particles, tubular structures or sheets of polymers, ceramics, glass and metals, carbon-based materials, and the like and combinations thereof. In case the strengthening element is micrometrically sized, it is called a composite material, or simply "cork composite". In case the dispersed phase is nanometrically sized, it is called a nanocomposite material or simply "cork nanocomposite".
In the present invention, the expression "cork material" or simply "cork" refers to particles, grains, granules or powder of natural cork, colmated natural cork, agglomerated cork, micro-agglomerated cork, recycled cork, cork waste, expanded cork, and the like or combinations thereof.
In the present invention, the strengthening elements may or may not enter the composition of the cork-based filling material formulations, used to fill the open pores of the cellular and porous metals or structures that give rise to the new materials and structures that are object of this invention.
The expression "cork-based material", in the context of the present invention refers to cork material as defined above combined with another material or materials such as, for example, a polymeric material of natural, synthetic or recycled origin.
The expression "polymeric material", or simply "polymer" refers to material of natural, synthetic or recycled origin, and the like and combinations thereof, that can be classified as thermoplastics, thermoset and elastomers according to their mechanical characteristics. Thermoplastics include the known plastics, which can be melted several times and in some cases dissolved in various solvents, thereby being recyclable materials. Thermoset are stiff and fragile polymers, being very stable to temperature variations, although heating these polymers causes them to decompose before melting, and therefore are not easy to recycle. Elastomers have a high elasticity and are neither rigid nor meltable, reducing the recycling ability.
The term "additive" or "processing additives" in the context of the present invention refers to substances used in small amounts employed for modifying and/or improving various properties, namely promoting processability, providing thermal stability, colouring, improving anti-static properties, surface hardness and fire resistance, among others.
The object of the present invention is to obtain porous and cellular metals and metallic structures of open porosity embedded with non-strengthened or strengthened cork bearing strengthening elements, which exhibit additional characteristics resulting from the combination of these cellular materials of metal and cork such as improved acoustic and thermal comfort as well as better mechanical performance, therefore being very versatile in their application.

Detailed description of the invention

The present invention relates to the development of porous and cellular metals and metallic structures of open porosity embedded with cork characterized by comprising at least two porous and cellular materials or structures, or cellular materials, wherein the first (1) comprises a matrix of a metal or metallic alloy, with periodically or randomly arranged open pores, and the second is (2) a natural cellular material, cork in the form of particles, granules, grains, powder, and the like and combinations thereof, which can be made of natural, expanded, recycled cork, cork waste from industrial processing, cork by-products, which cork can be strengthened with strengthening elements of natural, synthetic or recycled origin, namely but not limited to micro- or nanometric-sized metal, polymer or ceramics, glass and carbon, in the form of, e.g. but not limited to tubular structures, fibres, particles and combinations thereof, containing at least one synthetic, natural or recycled polymer, and optionally with the addition of processing additives.
In one embodiment, the porous and cellular metals and metallic structures of open porosity embedded with cork are characterized by comprising:

(a) one porous and cellular metal or metallic structure of open porosity obtained by any of the standard processes, including additive manufacturing technologies;
(b) one cork-based natural cellular material in the form of particles, granules, grains, powder, and the like and combinations thereof, which can be made of natural, expanded, granulated, recycled cork, cork waste from industrial processing with different grain sizes, cork by-products and cork waste, and the like and combinations thereof; and optionally
(c) one or several synthetic, natural or recycled polymers that may or may not undergo prior physical or chemical modification in order to improve the compatibility with the other components, possibly adding processing additives.

In one embodiment, the porous and cellular metals and metallic structures of open porosity embedded with cork composite are characterized by further comprising:

(a) 0.1% - 50% by mass of micrometric-sized strengthening elements of natural, synthetic, recycled origin, and the like and combinations thereof comprising, namely but not limited to fibres, particles, tubular structures or sheets, and the like and combinations thereof, made of metal, carbon, ceramics or polymer, and the like and combinations thereof.

In one embodiment, the porous and cellular metals and metallic structures of open porosity embedded with cork nanocomposite are characterized by further comprising:

(a) 0.1%-10% by mass of nanometric-sized strengthening elements of natural, artificial or recycled origin, and the like and combinations thereof comprising, namely but not limited to fibres, particles, tubular structures or sheets, and the like and combinations thereof, made of metal, carbon, ceramics or polymer, and possibly adding other micrometric-sized strengthening elements.

In one embodiment, the porous and cellular metals and metallic structures of open porosity embedded with cork composite are characterized by said cork-based material being selected from the group comprising particles, granules, grains, powder arising from natural, expanded, granulated, recycled cork, cork waste from industrial processing with different grain sizes, cork by-products and cork residues, and the like and combinations thereof.
In one embodiment, the porous and cellular metals and metallic structures of open porosity embedded with simple cork, cork composite or cork nanocomposite are characterized in that cork may or may not be previously subjected to treatment, such as physical or chemical modification, in order to improve the compatibility with the different components.
In one embodiment, the porous and cellular metals and metallic structures of open porosity embedded with simple cork, cork composite or cork nanocomposite are characterized by said polymer being selected from the group comprising natural, synthetic and recycled polymer, and the like and combinations thereof, such as thermoplastics, thermoset and elastomers, for example, polyethylene, polyurethane, silicone, epoxide, polypropylene, ethylene, alkyl or aryl anhydride, polystyrene, polycarbonate and the like and combinations thereof.
In one embodiment, the porous and cellular metals and metallic structures of open porosity embedded with simple cork, cork composite or cork nanocomposite are characterized by said polymer being optionally subjected to a physical or chemical modification in order to improve the compatibility with and uniform distribution of the different components, such as for example cork and strengthening elements, if any.
In one embodiment, the porous and cellular metals and metallic structures of open porosity embedded with cork composite or cork nanocomposite are characterized in that the strengthening elements are selected from the group comprising strengtheners of natural, synthetic, recycled origin, and the like and combinations thereof.
In one embodiment, the porous and cellular metals and metallic structures of open porosity embedded with cork composite or cork nanocomposite are characterized in that the strengthening elements are selected from the group comprising micrometric and/or nanometric scale fibres, particles, tubular structures or sheets, and the like and combinations thereof.
In one embodiment, the porous and cellular metals and metallic structures of open porosity embedded with cork composite or cork nanocomposite are characterized in that the strengthening elements are selected from the group comprising ceramics, metals and polymers, glass, carbon such as, for example, graphite, graphene, graphene oxide, carbon nanotubes, nano- or micrographite, nano- or microparticles or ceramic fibres such as silicon carbide, metallic fibres, glass fibres and polymer fibres, and the like and combinations thereof.
In one embodiment, the porous and cellular metals and metallic structures of open porosity embedded with cork composite or cork nanocomposite are characterized in that the micrometric or nanometric-sized strengthening elements are optionally subjected to physical or chemical modification in order to improve the compatibility with the different components and their uniform distribution within the cork matrix.
The invention also refers to the production process of porous and cellular metals and metallic structures of open porosity embedded with cork, a natural cellular material, either strengthened or not with strengthening elements, comprising the following steps:

a) Preparation and selection of raw materials;
b) Preparation of the strengthened or non-strengthened cork-based filling material;
c) Filling the open pores of a cellular or porous metal or structure with the strengthened or non-strengthened cork-based filling material; and
d) Densification and curing of the cellular material of metal and strengthened or non-strengthened cork.

The invention also refers to the uses of these new light-weighted and multifunctional materials in military, engineering and commercial applications. They are light-weighted, recyclable and reusable, and exhibit acoustic insulation properties, improved thermal properties compared to cork, excellent durability and excellent fire behaviour with absence or extinction of flame and release of toxic gases.
The present invention is useful for developing new porous and cellular materials and structures of metal and cork, obtained by embedding cork, a natural cellular material, in the open pores of a porous and cellular metal or metallic structure, possibly with the cork being strengthened with micro- or nanometric-sized strengthening elements, and the like and combinations thereof, which materials are light-weighted, recyclable, non-flammable, and present high mechanical and acoustic performance and improved thermal properties.
Advantages of this invention are the following, among others:

Obtaining new porous and cellular metals and metallic structures of open porosity embedded with strengthened or non-strengthened cork, which are light-weighted and multifunctional, recyclable and non-flammable for lightweight construction;
Obtaining new porous and cellular metals and metallic structures of open porosity embedded with strengthened or non-strengthened cork, which are light-weighted, for acoustic insulation and improved thermal conductivity compared to cork;
Obtaining new porous and cellular metals and metallic structures of open porosity embedded with strengthened or non-strengthened cork, which are light-weighted, present good absorption capacity of impact and shock energy, noise and vibrations, and can be used as core and filler for thin-walled tubular structures with different geometries and sandwich panels for structural applications, respectively.
Obtaining new porous and cellular metals and metallic structures of open porosity embedded with strengthened or non-strengthened cork, which are light-weighted, for design and furniture pieces, including the modern style due to their beauty and lightness.
Increasing the range of applications of porous and cellular metals and metallic structures of open porosity commonly used for functional applications, possibly also using them for structural applications since they are embedded with strengthened or non-strengthened cork that enhances their mechanical performance.
Possibility of using the waste of cork by-products, such as cork stoppers, expanded pure and black agglomerates through regranulation, cork waste from industrial processing with different grain sizes.
Recovering waste from the cork industry.

The following main applications can be highlighted:

Manufacturing furniture, decorative and design pieces;
Manufacturing light-weighted components for acoustic insulation and damping of noise and vibrations in the construction of tools, machines and devices;
Manufacturing light-weighted external and inner coating panels for acoustic and thermal insulation of buildings, houses and auditoriums, as well as in the automotive and aeronautical industries;
Incorporating these new metal and cork cellular materials as core and filler for sandwich panels and hollow structures for the transportation industry, in order to ensure a lightweight construction, acoustic insulation, damping of noise and vibrations and good absorption capacity of impact energy;
Manufacturing ballistic protection systems for personal protective equipment (e.g. vests), vehicles and houses.

Examples

Example 1. Preparation of a cork-embedded aluminium foam of open porosity and its compression behaviour

Cork-embedded aluminium foams of open porosity with an approximate density of 178.7 kg/m³ were prepared using granules of granulated cork with a particle size ranging between 0.5 mm and 1 mm ( Figure 2a).
An aluminium cellular metal of open porosity (25 × 25 × 25 mm) was prepared with an approximate density of 113.5 kg/m³ and pore size of 10 ppi (pores per inch) obtained by casting suspensions using the investment casting method.
In the first place, the cork-based filling material was prepared by mixing 1.5 g of granulated cork powders with a particle size ranging between 0.5 mm and 1 mm, with 10% by mass of polyurethane and 5% by mass of water, using a paddle mixer, for 5 minutes. Water was used as processing additive in order to promote the hardening of the polymer through cross-linking.
The resulting mixture was then poured into a stainless steel mould open at the top containing the aluminium foam of open porosity (25 × 25 × 25 mm) in its cavity (25 × 25 × 25 mm) that had been previously coated with a non-stick layer in order to favour the extraction of the resulting aluminium and cork foam.
After the filling process, the mould was closed at the top and the resulting cork-embedded aluminium cellular metal of open porosity was compressed just enough to ensure its densification, exerting a pressure that would not damage the initial aluminium foam of open porosity.
The closed mould containing the aluminium foam of open porosity was then placed in a preheated oven at 140°C for 2 hours. Finally, the cork-embedded aluminium foam of open porosity was extracted from the stainless steel mould.
For comparing the mechanical properties of the cork-embedded aluminium foams of open porosity, the following samples were prepared:

A. Cork agglomerate (25 × 25 × 25 mm) of 126 kg/m³, using the same methodology previously described but in this case the stainless steel mould was empty, and
B. Cork agglomerate with cork granules of size lower than 700 µm, where 20% (m/m) epoxide was used as adhesive material instead of polyurethane and without water added.

The mechanical properties (method described in example 1) of agglomerated cork with graphene oxide and nanocomposite hybrid structures were also assessed.
The process for modifying cork with graphene oxide (nanocomposites and nanocomposite hybrid structure) is as described in example 3, below.
The mechanical performance of samples of cork-embedded aluminium foam of open porosity produced according to this methodology was assessed via compression assays using a speed of 0.1 mm/s, according to ISO 13314:2011. The compression behaviour of these cork-embedded aluminium foams of open porosity was compared to the behaviour of their individual components (aluminium foams of open porosity and cork agglomerates). Three samples were tested for each type of material.
Figure 3 shows the average stress-strain ( Figure 3a) and mechanical energy absorbed by volume unit ( Figure 3b) curves for the different types of samples. The curve of mechanical energy absorbed by volume unit is obtained by integrating the stress-strain curve according to ISO 13314:2011.
The results clearly show that the developed cork-embedded aluminium foams of open porosity have better mechanical properties than their individual components, namely aluminium foams of open porosity and cork agglomerate samples ( Figure 3 ). The stress-strain curves ( Figure 3a ) and the respective curves of absorbed energy by volume ( Figure 3b ) of cork-embedded aluminium foams of open porosity are superior compared to aluminium foams of open porosity and cork agglomerates. For example, for a strain of 0.6, the stress and absorbed energy values for samples of cork-embedded aluminium foam of open porosity are 159% and 81% higher than the values for the initial aluminium foam of open porosity, respectively. It is also found that the yield stress value for the developed samples of cork-embedded aluminium foams of open porosity is much higher (about 0.344 MPa) than the yield stress value for agglomerated cork samples (about 0.116 MPa).
In the case of samples B (figure 3c) it can be seen that the presence of cork inside the aluminium foams results in increases of 395% in mechanical resistance for a strain of 0.6%.
The mechanical performance of the samples of cork-embedded aluminium foam of open porosity produced according to this methodology was assessed by quasi-static and dynamic compression assays using a speed of 0.1 mm/s and 284 mm/s, respectively, according to ISO 13314:2011, as can be seen in figure 3d.

Example 2. Preparation of cork-embedded aluminium foams of open porosity and their acoustic properties

Cork-embedded aluminium foams of open porosity 50 mm in diameter and 25 mm in height with a density of 230 kg/m³ were prepared using cork granules with a particle size lower than 700 µm, obtained by sieving the initial granules with a size distribution of 5 mm to 10 mm ( Figure 2a ).
To this end, aluminium foams of open porosity 50 mm in diameter and 25 mm in height ( Figure 2b ) were used with a density of 117.5 kg/m³ and pore size of 10 ppi (pores per inch), prepared by the investment casting method.
Firstly, a cork-based filling material was prepared by mixing 6.30 g of cork granules with sizes lower than 700 pm with 20% by mass of epoxide without using processing additives, employing a paddle stirrer, for 5 minutes.
7 g of the resulting mixture was poured into a cylindrical stainless steel mould open at the top with an inner cavity (50 mm in diameter and 25 mm in height) where the aluminium foam of open porosity 50 mm in diameter and 25 mm in height had been previously placed under mechanical vibration.
To favour filling the open pores of the aluminium foam of open porosity, the mould containing the aluminium foam of open porosity was placed on a vibrating platform. At the end of the pore filling process, the mould was closed at the top and the cork-embedded aluminium foam of open porosity (pores filled with cork) was compressed to ensure its densification, exerting a pressure not damaging the initial aluminium foam of open porosity. The mould with the resulting cork-embedded aluminium foam of open porosity was then placed in a preheated oven at 80°C for 2 hours. Finally, the cork-embedded aluminium foam of open porosity ( Figure 2d ) was extracted from the mould.
Samples of cork agglomerate 50 mm in diameter and 25 mm in height ( Figure 2c ) with a density of 158 kg/m³ were prepared from cork granules with a particle size lower than 700 pm using the same methodology described in the preparation of cork-embedded aluminium foams of open porosity, but in this case the cork-based filling material is poured into an empty mould with an inner cavity (50 mm in diameter and 25 mm in height), followed by pressing, curing (80°C and 2 hours) and extraction from the mould.
The acoustic performance and/or sound absorption efficiency of these aluminium foams of open porosity and their individual components were assessed according to ASTM E 1050. According to this standard, the test consists in placing a sample 50 mm in diameter and 25 mm thick of a given material in the inner border of an impedance tube 50 mm in diameter. The sound source, a noise generator RG10 emitting a random noise, is located at the other border.
There are still two microphones placed inside the tube between the sound source and the sample to be tested, measuring the pressure variations that the sound exerts on the specimen. The measured parameter is the sound absorption coefficient, defined as the property by which the materials can transform part of the incident sound energy in another form of energy (e.g. mechanical or thermal energy) . According to the standard, this property is defined as the sound absorption of a medium, being the sound power reduction by dissipation resulting from the propagation of sound in that medium. It depends on the type of surfaces, the angle of incidence of sound, the wave frequency and the application conditions of the system encompassing the material. Thus, α (alpha) is the relation between the amount of sound energy dissipated or absorbed by a given material and the incident energy on that material. It varies between 0 (0% absorption) and 1 (100%). The larger the % of absorbed sound, the greater the insulation effectiveness. This relation is quantified in a scale of 0 to 1, meaning that a material having a sound absorption coefficient of 0.5 absorbs 50% of the incident energy. Materials can be classified based on this property, and materials with coefficients equal to or greater than 0.5 are considered absorbent. Another parameter is the NRC indicator, the noise reduction coefficient, which is the arithmetical mean of the sound absorption coefficients α for the frequencies of 250 Hz, 500 Hz, 1000 Hz and 2000 Hz, rounded to multiples of 0.05. The results clearly showed that these cellular metals of aluminium of open porosity embedded with cork can be used for acoustic insulation, being highly effective noise absorbers over a wide range of medium-high frequencies (1000 to 3000 Hz). The performance is less impressive at low frequencies (below 1000 Hz), as shown in Figure 4a .
For comparison, aluminium foams made only of new composite samples were also prepared, with the addition of graphene oxide, according to examples 2 and 3.
Figure 4b compares the sound absorption and noise reduction coefficients for an aluminium foam embedded with graphene oxide-strengthened cork, named cork nanocomposite, and the respective individual components, namely aluminium foam and cork nanocomposite agglomerate, where it is found that the aluminium foam embedded with cork nanocomposite presents a high sound absorption coefficient over a wide range of medium-high frequencies (1000-4000 Hz), with the value of 1 between 1700 Hz and 2000 Hz and a value higher than 0.85 between 1261 Hz and 4000 Hz.
Figure 4c compares the sound absorption and noise reduction coefficients for an aluminium and cork foam and an aluminium foam embedded with graphene oxide-strengthened cork, named cork nanocomposite, and the respective individual components, namely aluminium foam, cork agglomerate and cork nanocomposite agglomerate, where it is found that the aluminium foam embedded with cork nanocomposite presents a high sound absorption coefficient over a wide range of medium-high frequencies (1000-4000 Hz), with the value of 1 between 1700 Hz and 2000 Hz and a value higher than 0.85 between 1261 Hz and 4000 Hz.

Example 3: Preparation of aluminium foams of open porosity embedded with cork and graphene oxide-strengthened cork and their thermal properties.

Aluminium foams of open porosity embedded with simple cork with a density of 230.4 kg/m³ and aluminium foams of open porosity embedded with graphene oxide-strengthened cork with a density of 223.8 kg/m³ were prepared using granules with particle sizes lower than 700 pm obtained by sieving the initial granules 5 mm to 10 mm in size.
To this end, samples of aluminium foams of open porosity (25 × 25 × 25 mm) with a density of 117.5 kg/m³ and pore size of 10 ppi (pores per inch) prepared by the investment casting method, were used. Commercial graphene oxide chemically exfoliated and marketed as a 0.4% m/m aqueous suspension was also used as cork strengthening element for preparing aluminium foams of open porosity embedded with graphene oxide-strengthened cork (cork nanocomposite).
The cork-embedded aluminium foams of open porosity (25 × 25 × 25 mm) were prepared using the same methodology described in example 2. 1.60 g of cork granules were mixed with 20% by mass of epoxide. The resulting mixture is poured into the open pores of a cellular metal of aluminium (25 × 25 × 25 mm) of 10 ppi that was inside the cavity (25 × 25 × 25 mm) of a stainless steel mould placed on a vibrating platform, followed by densification, curing (80°C for 2 hours) and extraction from the mould. Samples of cork agglomerates (25 × 25 × 25 mm) with a density of 154.6 kg/m³ were also prepared following this methodology, wherein the filling material was poured into an empty mould with a hollow cavity (25 × 25 × 25 mm) .
The aluminium foams of open porosity embedded with strengthened cork were prepared using graphene oxide as strengthening element. The first step of this process consisted in incorporating and evenly distributing the graphene oxide inside the cork granules using the layer-by-layer, LBL, deposition technique. To this end, the cork granules (< 700 µm) were immersed in successive solutions - 0.1% by mass aqueous solution of poly(diallyldimethyl-ammonium chloride); 0.1% by mass solution of Poly(sodium 4-styrene sulfonate) and 0.1% by mass solution of poly(diallyl-dimethylammonium chloride) - for 15 minutes, followed by filtering and washing with distilled water to remove impurities.
These cork granules were then immersed in a 0.1% by mass graphene oxide aqueous solution for 15 min, followed by filtering and washing with distilled water. The agglomerated cork granules containing the graphene oxide nanoparticles were dried in an oven at 40°C for 24 hours. 1.6 g of these granules were then mixed with 20% by mass epoxide. The resulting mixture was poured into the open pores of an aluminium foam of open porosity (25 × 25 × 25 mm) located inside a stainless steel mould that was placed on a vibrating platform under vibration, followed by densification, curing (80°C for 2 hours) and extraction from the mould.
Likewise, samples of graphene oxide-strengthened cork (25 × 25 × 25 mm) with densities of 154.9 kg/m³ were prepared using the same methodology described for the preparation of aluminium foams of open porosity embedded with graphene oxide-strengthened cork, wherein the mould was empty.
The thermal conductivities of the different samples were measured ( Figure 5) using a Hot Disk equipment, model TPS2500, according to ISO 22007-2. The measurement is performed using a transient flat area sensor placed between two identical specimens of the same type of material. This method consists in applying an electrical current intensity and measuring the heat propagation resistance of the sample, i.e. recording the temperature profile inside the sample (axial and radial) as a function of time.
The results ( Figure 5) clearly show that the aluminium foams of open porosity are the ones presenting the highest value ([0.178 W/(m.k)]), followed by the cork-embedded aluminium foams of open porosity ([0.091 W/(m.k)]) and the ones strengthened with graphene oxide ([0.101 W/(m.k)]). These samples have higher values than the samples of non-strengthened agglomerated cork ([0.055 W/(m.k)]) and graphene oxide-strengthened agglomerated cork ([0.057 W/(m.k)]). These aluminium foams of open porosity embedded with strengthened or non-strengthened cork exhibit thermal conductivity values that make them suitable for applications where it is important to have values higher than cork.
There are materials on the market that are used as thermal insulators with thermal conductivity values similar or even higher than these aluminium foams of open porosity embedded with strengthened or non-strengthened cork.
A good thermal insulation must have not only a low thermal conductivity but also a good thermal diffusion so that outside temperature variations are not easily transferred to indoor spaces. Furthermore, they must be chemically inert, dimensionally stable and easy to apply on surfaces. The advantages of the resulting aluminium and cork foams are their low density, ability to withstand high temperatures and good resistance to compression. These new cellular metals of aluminium of open porosity embedded with strengthened or non-strengthened cork are important in applications where an improvement of these thermal properties, while maintaining good acoustic insulation, is intended.

Example 5: Preparation of aluminium foams of open porosity embedded with graphene oxide-strengthened cork and their flame retarding and extinguishing behaviour

Samples of aluminium foams of open porosity embedded with graphene oxide-strengthened cork and of graphene oxide-strengthened cork agglomerates were prepared using the methodology described in example 4. Figure 6a shows a sequence of images from the flame test performed for each type of sample. The images were taken every 2 seconds. The flame extinction time of the samples was determined by subjecting the samples to a flame from a candle for 5 seconds and then measuring the time the flame took to extinguish completely. The results show that the flame is extinguished faster in the aluminium foams of open porosity embedded with graphene oxide-strengthened cork.
Figure 6b refers to a sample (25 × 25 × 25 mm) of cork agglomerate, a sample (25 × 25 × 25 mm) of aluminium and cork foam, a sample (25 × 25 × 25 mm) of graphene oxide-strengthened cork agglomerate, referred to as cork nanocomposite, and a sample (25 × 25 × 25 mm) of aluminium foam embedded with cork nanocomposite, and it can be seen that the flame is extinguished faster in the aluminium foams of open porosity embedded with graphene oxide-strengthened cork.

References

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Claims

Porous and cellular metals and metallic structures of open porosity embedded with cork, characterized by comprising:
- at least a porous or cellular metal or a porous or cellular structure with open, interconnected, edge-sharing pores, resulting in three-dimensional arrays periodically or stochastically arranged, thereby forming a metallic matrix foam of at least one metal or metallic alloy, and

- a pore-filling material of said cork-comprising matrix, wherein the cork is present in the form of particles, grains, granules or powder with different densities and/or grain sizes, whose size distribution is smaller than the open pores of said porous and cellular metal and metallic structure.
Porous and cellular metals and metallic structures of open porosity embedded with cork according to claim 1, characterized in that the metal is selected from aluminium, magnesium, manganese, copper, silicon, zinc, tin, nickel and alloys thereof.
Porous and cellular metals and metallic structures of open porosity embedded with cork according to any of the preceding claims, characterized in that the filling material comprises cork from recycled cork or cork waste from industrial processing.
Porous and cellular metals and metallic structures of open porosity embedded with cork according to any of the preceding claims, characterized in that the metal open foams have pore sizes ranging between 1 ppi and 500 ppi, more preferably of 10 ppi to 250 ppi, even more preferably of 50 ppi to 150 ppi, or still 75 ppi to 100 ppi.
Porous and cellular metals and metallic structures of open porosity embedded with cork according to any of the preceding claims, characterized in that the filling material comprises at least one strengthening element selected from ceramics, metals, polymers, glass, carbon-based materials and the like in the form of particles, fibres, tubular structures or sheets, and the like and combinations thereof.
Porous and cellular metals and metallic structures of open porosity embedded with cork according to any of the preceding claims, characterized in that the polymer filling material is polyurethane or epoxide.
Porous and cellular metals and metallic structures of open porosity embedded with cork according to any of the preceding claims, characterized in that the filling material contains 0-50% by mass of micrometric-sized strengthening elements, or 0-10% by mass of nanometric-sized strengthening elements.
Porous and cellular metals and metallic structures of open porosity embedded with cork according to any of the preceding claims, characterized in that the filling material comprises processing additives, preferably of the cross-linking type.
Process for the production of porous and cellular metals and metallic structures of open porosity embedded with cork as described in claims 1 to 8, characterized by comprising the following steps:
a) Preparation of the cork-based filling material by mixing the selected starting materials, in particular the cork material with the polymer, and optionally incorporating at least one strengthening element;

b) Filling the open pores of one cellular or porous metal or structure with the filling material of (a), by pouring that material into the open pores of a metallic matrix of at least one metal or metallic alloy; and

c) Densification and curing of the material from (b).
Process according to claim 9, characterized in that, in step (a) the strengthening element is incorporated through dry and/or wet techniques, using mechanical stirrers, by colloidal processing and step-to-step technique.
Process according to claim 9 or 10, characterized in that, in step (b) the pore-filling operation is conducted under vibration and/or pressure.
Process according to claim 9, 10 or 11, characterized in that, in step (b) thin-walled moulds or hollow structures with simple and/or complex geometries are used.
Process according to the previous claim, characterized in that, in step (b) the thin-walled moulds or hollow structures with simple and complex geometries have a coating with non-stick properties.
Process according to the previous claim, characterized in that the coating with non-stick properties is tracing paper, Teflon sheets, Kraft paper and/or silicone-based paper and films.
Process according to claim 9, 10, 11, 12, 13 or 14, characterized in that step (c) is conducted in air or in vacuum, with no temperature applied or also possibly varying the temperature, typically up to 250°C.
Use of the porous and cellular metals and metallic structures of open porosity embedded with cork as described in claims 1 to 8, characterized by being applied in the industry of porous and cellular metals and metallic structures of open porosity, namely in metallic foams and metallic sponges.
Use of the porous and cellular metals and metallic structures of open porosity embedded with cork as described in claims 1 to 8, characterized by being applied in the construction of houses, cars, trains, boats, aircraft, tools, machines, devices, furniture, design pieces, for acoustic insulation, acoustic absorption, lightweight construction, with improvement of thermal behaviour and impact and shock energy.
Use of the porous and cellular metals and metallic structures of open porosity embedded with cork as described in claims 1 to 8, characterized by being applied as core or filler for sandwich panels or hollow, thin-walled tubular structures for impact energy absorption systems and ballistic protection systems intended for vehicles, houses and personal protective equipment, such as vests.